That happens due to the varies height of a post. YOu can set the minimum height of the post like this;
Note: If still you are getting any issue then please increase the min height value.
And you are using page builder as page template, plese switch it to full width page templates.
We had reviewed your and found that you are using old version of flat responsive. We are already on to flat responsive 4.2.0. YOu are using version 3.5.0. And please make a back up of your site before updating. So you can take a action if disaster occurs.